Throughout the present description and the subsequent claims, the gearshift to which reference is made can either be the rear gearshift, which moves the chain between the different sprockets associated with the rear wheel of the bicycle, and the front gearshift, which moves the chain between the different crowns or chain rings associated with the pedal cranks.
Usually, the actuator device of the gearshift comprises an articulated quadrilateral mechanism intended to be integrally fixed to the bicycle frame and to the derailleur of the gearshift; the deformation of the articulated quadrilateral causes the displacement of the derailleur thereby causing a gear shift.
The aforementioned deformation of the articulated quadrilateral can be obtained manually, through the movement of control levers transmitted to the actuator through cables of the Bowden type, or else with an electric motor that—following an appropriate command imparted by the cyclist and through a suitable mechanism—displaces different parts of the articulated quadrilateral with respect to each other, deforming it and thus displacing the derailleur.
Devices of this type, and in particular actuated through an electric motor, are known in the art.
The actuator devices of the prior art, although substantially meeting the constant demands of gearshift producers to improve the precision of actuation, upon which the ease and reliability of operation of the gearshift depends, do, however, have some drawbacks still not solved.
In particular, the Applicant has noted that in the above mentioned actuator devices equipped with electric motors, the axial component of the force applied by a drive shaft to the transmission elements causes an equal and opposite axial thrust on the motor, which, if not suitably countered, tends to make it move axially with respect to the support element that houses the motor itself. This effect is emphasized in certain hot or cold environmental conditions, in which the different expansion coefficient of the materials constituting the casing of the motor and the support element further worsen the coupling between motor and support element. An undesired translation in the axial direction of the motor with respect to the support element causes a corresponding translation of the drive shaft with respect to the transmission elements, and therefore a worsening of the accuracy of actuation of the derailleur. Undesired axial translations can also be caused by accidental knocks on the gearshift.
The technical problem underlying the present invention is therefore that of providing an actuator device for a bicycle gearshift of the type described above, in which the coupling between motor and support element is such as to prevent mutual translations in the axial direction in any operating or climatic condition.